1. Field of the Invention
The invention relates to an ink jet print head with mutually parallel ducts formed inside a substrate and separated by partition walls. The ducts are provided with a cover plate and one outlet opening on each of their ends. One thermal or piezoelectric element is associated with each duct. Upon excitation and with ink fluid disposed inside the duct, the element effects an expulsion of a drop of ink from the outlet opening. The invention further relates to a method of producing such an ink jet print head.
2. Description of the Related Art
Ink jet print heads are widely used in ink jet printers. The ink jet print head usually operates by the known drop on demand or DOD method, described for instance in German Patent DE 30 12 698 C2. There, to create a dot on a medium to be imprinted, such as paper, a drop of ink is expelled from a duct of the ink jet print head as soon as a thermal or piezoelectric element associated with the duct is triggered with a suitable current pulse from a driver circuit. The excitation occurs as the result of a current pulse 2 xcexcs to 10 xcexcs in duration, for instance, thus releasing thermal energy of approximately 15 to 50 microjoules. This heating leads to local evaporation of the ink fluid (bubble formation). The column of fluid is positively displaced from the corresponding duct outlet opening but without initially tearing. Once the current pulse ends, the bubble collapses above the thermal element. As a consequence, some of the fluid column is drawn back in. A drop of ink separates from the column outside the duct outlet openings and moves onward due to the conservation of momentum. These drops of ink create a black printed dot, in the case of black ink, on the paper. The typical emission frequency is approximately 5 kHz.
To create a character, such as a letter, the thermal or piezoelectric elements of the parallel ducts must be suitably supplied with current pulses by the driver circuit in such a way that the dots required for these letters become visible on the paper as a result of the impact of corresponding drops of ink.
Because of the very small duct diameter and close matrix spacings between the ducts (or jets), processing methods known from semiconductor technology are employed to create ink jet print heads. Examples of such processing methods are described in European Patent Disclosures EP 0 359 417 A2 and EP 0 434 946 A2, and in IEEE Transactions on Electron Devices, Vol. 26, 1979, p. 1918. In contrast to the production of integrated semiconductor circuits, which are formed on a single substrate, the prior art methods for producing ink jet print heads require at least two different substrates. On one substrate, partitions between ducts are formed, and these are closed by a separately produced cover plate made of a second substrate.
In the prior art methods, heating resistors can be disposed on or in the duct for thermal excitation. Often the ducts are formed by orientation-dependent etching in a silicon substrate. The heating resistors can be secured to the ducts by bonding. A glass plate, for instance, may be used as the cover plate. The glass plate is mounted on the duct plate, and hence in the first substrate, by anodic bonding.
As disclosed by the European document EP 0 443 722 A2, the ducts of the ink jet print head can also be formed by adjusting a cover plate, provided with partitions, onto a first substrate that is provided with heating resistors. Instead of the cover plate provided with partitions, a flat cover plate can also be glued to the first substrate, if the aforementioned ducts have already been machined into the first substrate, in the form of duct bottoms and two duct side walls each. The glued-on cover plate then forms the top of the duct for these ducts.
A problem associated with these prior art methods for producing integratable ink jet print heads is the absolute necessity of two substrates that must be joined to one another. This requires complicated adjustment, and the fine conduits must be protected against contamination while the two substrates are being glued together, which means additional effort and expense.
It is accordingly an object of the invention to provide a TITLE, which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type and which renders unnecessary complicated adjustment and gluing and bonding of two separately produced substrates.
With the foregoing and other objects in view there is provided, in accordance with the invention, an ink jet print head, comprising:
a substrate formed with a plurality of mutually parallel ducts each having an outlet opening and partition walls separating the ducts;
an ink ejection element, selected from the group consisting of thermal elements and piezoelectric elements, operatively associated with each of the ducts for selectively ejecting ink fluid from the ink duct and ejecting ink droplets through the respective outlet openings upon an excitation of the ink ejection element; and
a cover plate disposed on the ducts, the cover plate including a first layer disposed directly on the ducts, the first layer being a deposition layer formed with a plurality of openings, and including a second layer disposed directly on the first layer and covering the openings, the second layer being a deposition layer formed by depositing a material selected from the group consisting of boron phosphorus silicate glass and Si3N4.
In other words, the above-noted objects are satisfied in that the cover plate comprises at least two layers, the cover layer is disposed directly on the duct with its first layer, the first layer is formed with a plurality of openings located above the ducts, and a second layer closing the openings is formed directly on the first layer (on its surface remote from the duct.
In accordance with an added feature of the invention, an electronic trigger circuit integrated inside the substrate.
In accordance with an additional feature of the invention, the thermal elementxe2x80x94a heating resistor formed by a polysilicon layerxe2x80x94is disposed on the bottom of the duct. One or more protective layer may be disposed between the duct bottom and the polysilicon layer.
In accordance with another feature of the invention, the ink ejection elements are disposed inside the duct and suspended peripherally from the side walls of the ducts. In that case, the ink ejection elements are formed of erosion-proof material.
When the ink ejection elements are chemical elements, the invention provides for a heat-storing layer disposed below the chemical element distally from the duct bottom. The preferred heat-storing layer is a layer of silicon oxide with a thickness greater than 1.0 xcexcm.
In accordance with a further feature of the invention, at least one protective layer is disposed between the duct bottom and the ink ejection elements when they are formed of thermal elements. The protective layer is formed with a plasma oxide layer and a plasma nitride layer. Preferably, the plasma oxide layer has a thickness of substantially 300 nm and the plasma nitride layer has a thickness of substantially 600 nm.
In accordance with again an added feature of the invention, a second protective layer is formed on the first above-mentioned protective layer. That second layer is preferably a sputtered tantalum layer.
In accordance with again an additional feature of the invention, the ducts have side walls with a height between substantially 5 xcexcm and substantially 50 xcexcm. The side walls may be formed of plasma oxide, polysiloxanes, or polyimide. The first layer of the cover plate may be a layer of structured plasma nitride and structured polysilicon, and the second layer may be formed of boron phosphorus silicate glass or Si3N4.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing an ink jet print head, the method which comprises:
providing a substrate and placing ink ejection elements at locations of the substrate where ink ducts of the ink jet print head are to be formed, the substrate defining side walls of the ducts to be formed;
depositing a first layer on the substrate;
structuring the first layer with a multiplicity of openings above locations where the ink ducts are to be formed;
isotropicaly etching the substrate through the openings in the first layer until a plurality of ducts have been etched in the substrate;
depositing a second layer on the first layer and closing the openings; and
forming each of the ducts with an outlet opening at a respective end thereof.
In accordance with yet an added feature of the invention, the substrate is deposited as a plasma oxide, polysiloxanes, and polyimide with a thickness of between substantially 5 xcexcm to 50 xcexcm onto a base plate. The first layer is structured photolithographically with subsequent dry etching.
The ducts are preferably etched out of the substrate with an isotropic etching process by dry etching with a fluorine-containing plasma in HF steam or by wet etching with BHF. Where the substrate is formed of organic material, such as polyimide, isotropic etching is with O2 plasma.
The second layer may be boron phosphorus silicate glass deposited by CVD deposition or it may be formed by plasma-Si3N4 deposition. After the second layer is deposited onto the first layer, a flow process may be performed at high temperatures.
The substrate may be formed by the following sequence: in a first step, depositing the substrate to approximately half a desired thickness of the substrate; in an ensuing step, applying a resistance layer and structuring the resistance layer; and in a further step, depositing a second half of the substrate onto the resistance layer. The resistance layer will be exposed directly to the ink in the ducts and, accordingly, it will be formed as an erosionproof layer.
Openings in the first layer at the ends of each of the ducts should be large enough so that an ensuing operation of depositing the second layer does not close the given openings. Those large opening then form the outlet openings.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an ink jet print head and method for producing such an ink jet print head, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Specifically, the ink jet print head of the invention and its production method will be described in further detail below in conjunction with exemplary embodiments. In the exemplary embodiments, the ink jet print head and its production method will be described in terms of a print head with thermal excitation. However, it is equally possible to produce a print head with piezoelectric excitation. The invention therefore also relates to such print heads with piezoelectric excitation.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.